That means the MOST RECENT individual that lived that stands in the
direct line through mothers with the set of individuals in question. If
we are looking at the entire population of people today, the MRCA of that
set is called mitochondrial Eve. The matrilineal MRCA of humans is estimated
to have lived between 150,000 to 200,000 years ago. (note that there are
other MRCAs; eg the MRCA as measured by the X-chromosome - since the
X-chromosome descends through fathers and mothers, the lineage is different
- but there is still the concept of the individual from whom the
X-chromosomes of all people alive today descended. That is not the same as
matrilineal MRCA or Mitochondrial Eve).